As mentioned in U.S. Pat. No. 4,077,467, conventional methods for soldering, fusing, or brazing articles are generally not appropriate when a large number of such operations are to be performed rapidly, as in a mass production process. Thus, for example, when mass soldering printed circuit boards having many closely spaced connections, conventional soldering iron techniques are of little use when rapid soldering must be done. Several methods have been proposed in the past which utilize hot, saturated vapors. When these vapors come into contact with the article to be processed, they melt the solder or brazing material. One such method is described in U.S. Pat. No. 3,866,307 in which the vapors condense on the article and give up latent heat of vaporization to heat the article to the temperature needed for soldering, fusing, or brazing. To implement such a process an apparatus for performing the desired operation can be arranged to permit a single article or a batch of articles to be subjected to hot, saturated vapors in a suitable enclosure. The heat transfer liquid, for example, may be selected from a group of liquids known generically as fluorocarbons.
Systems presently in use utilize either a single vapor or a primary vapor and a secondary vapor blanket which prevents the loss of the primary vapor. In order to accomplish this and in accordance with the teaching of U.S. Pat. No. 3,904,102 and the aforementioned U.S. Pat. No. 3,866,307, the primary and secondary vapors are supplied from a mixture of primary and secondary liquids in the same reservoir. The secondary vapors boil off first at a lower temperature and fill the upper portion of the open enclosure. The more dense primary vapors which boil at a higher temperature fill the lower portion thereof. Problems in the regulation and maintenance of such multiple vapor systems are addressed in U.S. Pat. No. 4,077,467 and while such systems work satisfactory for most purposes, the maintenance of a multiple vapor system is oftentimes difficult and optimization of system operation is more difficult than utilizing a single vapor.
It will be appreciated that the vapors utilized are exceedingly costly and the recovery of such costly vapor is of paramount importance in the provision of an industrially useful process.
U.S. Patents relating to single or multiple vapor soldering, fusing, or brazing are Nos. 2,515,489; 3,866,307; 3,904,102; 3,947,240; 4,022,371; 4,032,033; 4,055,217; 4,077,467; 4,090,843; 4,115,601; 4,194,297, with vapor recovery systems being described in U.S. Pat. Nos. 2,582,789; 3,078,701; 4,012,847; and 4,029,517.
Most notably, U.S. Pat. No. 4,264,299 describes a two zone single vapor system in which the processed article is drained in the vapor processing zone. It is the primary object of this patent that drainage occurs in the vapor processing zone before the article is moved to an upper zone for evaporation and removal of any vaporized working fluid.
In accordance with the teachings of U.S. Pat. No. 4,264,299 and an article entitled "Design Considerations for a Closed, Single Fluid Vapor Condensation Machine", E. J. March and J. D. Nannis, pps. 501-511, published in Proceedings of the Technical Program, National Electronic Packaging and Production Conference, Anaheim, Calif., February, 1981, it was thought that draining in the vapor processing zone was necessary so that evaporation could efficiently take place in the upper zone due to the latent heat of the article. One reason draining was performed in the vapor processing zone was the suposition that a layer of condensed liquid on the article would interfere with the evaporation and recovery process in that the sensible heat of the article would be insufficient to cause a reevaporation of the condensed vapor on the article when the aritcle was moved to the upper zone. Not only was it thought that drainage would have to occur in the vapor processing zone, but also that vapor-laden gas in the upper zone would have to be heated prior to recirculation back to the upper zone in order to promote evaporation. It should be noted that in vapor soldering and degreasing machines made in accordance with the teachings of the 4,264,299 patent and aformentioned article, the vertical rise of the second zone was made high enough to assure complete drainage prior to re-evaporation in the upper zone.